(1) Field of the Invention
The present invention relates to a fixing device for fixing an unfixed image formed on a recording sheet by applying heat and pressure to the unfixed image while the recording sheet is passing through the fixing nip, the fixing nip being formed by pressing a fixing roller and a pressure belt against each other. The present invention also relates to an image forming apparatus having the fixing device.
(2) Description of the Related Art
Image forming apparatuses such as photocopying machines are equipped with a fixing device for fixing an unfixed image formed on a recording sheet, such as a sheet of recording paper and an OHP sheet, by applying heat and pressure. Literature 1 (Japanese Patent Application Publication No. 2005-331576) discloses a belt nip type fixing device in which a fixing nip is formed between a roller and a belt. FIGS. 11-15 are schematic views illustrating the technology relating to the Literature 1. FIG. 11 is a schematic cross-sectional view illustrating the structure of the fixing device disclosed in the Literature 1.
In this fixing device, a fixing nip N is formed by pressing a fixing belt 62 against the surface of the heat roll 61, which is provided in parallel with the heat roll 61, toward the axis of the heat roll 61. The heat roll 61 rotates with the surface heated to a predetermined temperature. A pressure pad 63 for pressing the fixing belt 62 against the heat roll 61 is provided inside the area that the fixing belt 62 rotates around. The fixing belt 62 is pressed by the pressure pad 63 against the heat roll 61 which is driven to rotate, and accordingly the fixing belt 62 rotates as the heat roll 61 does. A heater 67 is provided along the axis of the heat roll 61.
The pressure pad 63 is attached to a supporting frame 65 via an elastic sheet 66. The supporting frame 65 is provided inside the area that the fixing belt 62 rotates around, and is biased toward the heat roll 61. By the biasing force (pressing force) applied to the supporting frame 65, the pressure pad 63 presses against the inside surface of the fixing belt 62, along the width direction of the fixing belt 62 (i.e. the axis direction of the heat roll 61).
A low friction sheet 64 for reducing the sliding friction between the pressure pad 63 and the fixing belt 62 is provided between them. Although FIG. 11 depicts the low friction sheet 64 and the fixing belt 62 as though they are away from each other and the pressure pad 63 does not press against the low friction sheet 64, this is for clearly showing the structures of the low friction sheet 64, the fixing belt 62 and the pressure pad 63. In reality, in a fixing device that has been built up, the low friction sheet 64 is pressed by the pressure pad 63 against the inside surface of the rotating fixing belt 62 such that the low friction sheet 64 slides on the inside surface of the rotating fixing belt 62.
The pressure pad 63 includes an elastic pressing part 63a and a rigid pressing part 63b. To the rigid pressing part 63b, the biasing force of the supporting frame 65 is applied via the elastic sheet 66. The elastic pressing part 63a is supported by the rigid pressing part 63b so as to press the inside surface of the fixing belt 62 via the low friction sheet 64. The elastic pressing part 63a consists of an elastic body. The rigid pressing part 63b is made from a rigid material that is harder than the elastic pressing part 63a. 
FIG. 12 is a cross-sectional view showing the structure of the pressure pad 63, and FIG. 13 is a perspective view of the same. By the biasing force (pressing force), which is applied to the rigid pressing part 63b and transmitted to the elastic pressing part 63a, the elastic pressing part 63a is pressed against the inside surface of the fixing belt 62 via the low friction sheet 64. The rigid pressing part 63b has a rigid presser 63c, which is disposed downstream in the rotation direction of the fixing belt 62. The rigid presser 63c, provided along the elastic pressing part 63a, projects toward the heat roll 61. The edge of this projection of the rigid presser 63c presses against the inside surface of the fixing belt 62 via the low friction sheet 64.
The low friction sheet 64, which is pressed against the fixing belt 62 by the pressure pad 63, is configured to have a rectangular shape. The low friction sheet 64 is disposed along the width direction of the fixing belt 62 such that the downstream end of the low friction sheet 64, which is downstream from the other end in the rotation direction of the fixing belt 62, is positioned downstream from the rigid presser 63c of the rigid pressing part 63b. The width of the low friction sheet 64 is almost the same as the width of the fixing belt 62.
The low friction sheet 64 extending from its downstream end passes between the pressure pad 63 and the fixing belt 62, and bends away from the inside surface of the fixing belt 62 along the upstream lateral side of the pressure pad 63 with respect to the rotation direction of the fixing belt 62. The upstream end, 64b, of the low friction sheet 64 is inserted between the rigid pressing part 63b and the elastic sheet 66.
FIG. 14A is a front view of the upstream lateral side of the pressure pad 63. FIG. 14B is a developed view of the upstream end 64b of the low friction sheet 64. The face (i.e. rear face) of the rigid pressing part 63b that faces the elastic sheet 66 is provided with a plurality of locking parts 63d that project toward the elastic sheet 66. The locking parts 63d are arranged with predetermined gaps, along the width direction of the fixing belt 62. The upstream end 64b of the low friction sheet 64, which is inserted between the rigid pressing part 63b and the elastic sheet 66, is provided with a plurality of locking holes 64a that correspond to the locking parts 63d of the rigid pressing part 63. The locking parts 63d are to be inserted into the locking holes 64a in one-to-one correspondence.
As the locking parts 63d are inserted into the locking holes 64a, the upstream end 64b of the low friction sheet 64 is fixed to the rigid pressing part 63b. The low friction sheet 64, extending from the upstream end 64b, is pulled out along the lateral side of the rigid pressing part 63b, and is bent to pass between the pressure pad 63 and the fixing belt 62.
In the fixing device having such a structure, the pressure pad 63 presses against the inside surface of the fixing belt 62 via the low friction sheet 64, and therefore the low friction sheet 64 slides on the inside surface of the fixing belt 62 while being pulled downstream by the rotating fixing belt 62. The low friction sheet 64 is made of a material that does not produce a large friction either on the fixing belt 62 or on the pressure pad 63. Thus, the low friction sheet 64 reduces the sliding friction between the fixing belt 62 and the pressure pad 63.